A standard adjustable firecode door casing is formed by a pair of upright side elements and a horizontal lintel element bridging the side elements, the elements each in turned being formed by an inner and an outer profile. The inner profile is formed with the doorstop. The two profiles have flanges that are directed toward each other and the flange of the inner profile is normally of U-section so that the outer-profile flange can slip into it. Bolts engaged between the two profiles secure them together to either side of the wall edge they are mounted on.
One of the problems with such door-casing elements is that they have only a limited degree of adjustability, normally at most one inch. Thus the builder must buy that door frame whose range covers the wall thickness in question, which often leads to having to buy several different sizes of frames in order, for instance, to compensate for the thicker walls of lower stories.
In addition it is essential in firecode construction that the profiles be made of steel. Aluminum would be preferred, since it does not rust, is lighter, and can be produced with attractive hard finishes, but aluminum is a better conductor of heat so that it softens much sooner than steel would. In fact it is impossible to make a so-called one-hour door casing, that is a casing which will hold out fire for one hour, of aluminum with known technology. What is more the complex profiles needed can be produced relatively easily by extrusion in aluminum, but can only be made in steel with expensive high-technology equipment.